Preheat oven to 350F. Line a 9x13” pan with foil and spray well with cooking spray. These are very sticky and the foil helps remove them easily from the pan, but it’s not mandatory.
Cream butter and sugars in the bowl of an electric mixer fitted with a paddle attachment. Beat in egg and vanilla. Mix in baking soda and cinnamon. Slowly beat in flour. Mix in oats.
Press dough into pan. Dough will be sticky so it helps to spray your hands with cooking spray. It will be a thin layer on the bottom of the pan. Bake for 10 minutes.
Remove hot pan from oven and sprinkle evenly with toppings. Pour sweetened condensed milk evenly over the top. Return to oven and bake for 25-30 more minutes until the edges are browning and the top seems semi-set. These stay sticky, and sometimes the chips (especially butterscotch) melt around the edges.
They may look very brown, but it’s just the melted chips. Cool completely before cutting.